Are JET and ITER really any LESS stable than the SUN??

[Chris Bradley]

I just wanted to cover one point I suggested in another post, which was that a star (that is, the astrophysical type rather than Australian!) is probably the only true way of getting a continuous fusion reaction.

I don't think this tells the whole story, because I don't automatically accept that solar fusion is, actually, any more stable than magnetic confinement systems here on earth. It may look that way to us puny little short-lived creatures, but we're talking astronomical scales here!

I arrived at this conclusion a couple of years ago with the following order-of magnitude calculation and assumptions:

Assumptions; 1) Fusion plasma instabilities are caused by MHD vorticity in the plasma.

2) The total amount of vorticity is proportional to the volume. More volume gives more space for vortices of some particular size to form.

3) Instability occurs when vortices interact with each other, so the further they can be apart then the more stable is the plasma.

4) Final 'catastrophic' instability is caused as interacting votices reach the surface of the plasma and the surface is breached.

If these assumptions are correct, we can therefore calculate a stability factor of a fusion plasma by: (volume * max. dimension)/(surface area). That is, as volume and/or total dimensional extent go up, stability goes up, as surface area comes down, so stability comes down. This seems quite logical to me and not particularly contentious.

Now let's calculate for the sun: (1.4E27m3 * 1.4E9m)/(6E18m2) = 3.3E17

And for JET: (170m3 * 6m)/(200m2) = 5

These have units of m2, but disregarding this, if you now look at how long the sun's main sequence is, it goes red-giant in 10 billion years. This is [quite co-incidentally??] about 3E17 seconds.

And as we all know, JET has 'stability' for just a few seconds.

The conclusion: ITER's maximum period of stability will be: (840m3 * 12.4m) / (820m2) =eqivalent= 12.7 seconds, and that if better than this, then it is actually more stable than the Sun itself.

best regards,

Chris MB.

[Richard Hull]

An interesting bit of work.

The only thing that makes the Sun stable over geologic time is the overburden of terrible fusion fuel that won't burn except in the core. (if you believe in p-p fusion) Gravity is the confining force, unburnable fuel the containment.

What goes on in the core is truly unknown. I don't hold with p-p fusion, personally. I feel it is dreamed up to get to the real fuel d-d and d-t.

I have always held to p-e fusion to n and then n-p fusion to d and from there off to the races. Make neutrons in the right environment and matter, as we know it, is easy to assemble.

Richard Hull

[Chris Bradley]

Are you not really saying much the same thing as 'conventional wisdom'?

I thought the p-p reaction was moderated by the weak force, namely the mutation of a proton into a neutron by the emission of a positron, which happens very rarely indeed, hence the diminishingly poor unit volume stellar power output.

p + [p] -> p + [n + e+] -> D + e+

Whether a proton emits a positron or absorbs an electron seems to be saying pretty much the same thing when contemplating the unseeable interior of a star several dozen million miles away!

best regards,

Chris MB.

[Richard Hull]

Semantics are one thing, physical facts that actually embody a process another. No one has seen what goes on in the core of a star, only theorized and hypothesized. Any number of theories can exist, but many are untestable on earth.

The p-p process has the advantage of being rubber stamped with a now rather old nobel prize, in spite of no one seeing into or experimenting within the core of a star.

The p-p process is an exothermic one and to avoid overheating a star it is given a 1 in 10e25 chance of happening (convenient). This allows for a slow burning main sequence star.

The p-e process is endothermic and would tend to cool the star's interior save for future easy fusions once D and T are formed from p-n fusion. Also a possible slow burn process.

Regardless, neutrons are the key to making D and deuterium is the real fuel.

Richard Hull

[Chris Bradley]

Do you have any more info on this p+e- reaction? How do you determine its reaction rate (no Coulomb barrier to jump)?

I know nothing about it (only 'conventional' rubber stamped books on astrophysics on my shelf).

[Richard Hull]

I am sorry to say that we only have that which is supplied us by theory. We do know that the neutron is more massive than its constituents and this tells us that energy had to be added to affect its formation. (endothermic formation).

The neutron is completely unstable outside of the nucleus breaking up into a proton and an electron plus excess energy within 10-20 minutes on average, yet stable over geologic time within nuclei that are not themselves excited or unstable.

This means there are no neutrons in space, anywhere save near stars or violent plasmas. They are only found in bulk matter in a totally stable state.

Neutrons are produced in stars, this we know. They suck energy from the fusion process of their formation and store it as binding energy. They are a special item as most all low z materials fuse exothermically. Some few charts of the nuclides now include the neutron as the first nuclide and not hydrogen.

Electron capture in radioactive nuclei is said to create nuclear neutrons outside of a fusion furnace by fusing with an excess nuclear proton.

Interestingly, we have never seen a neutron in a nucleus, we suppose they exist there, nonetheless.

It is important to realize that most any mainline scientist will tell you that a neutron is not made up of a cojoined proton and an electron....The spins and magneitc moments aren't right. This, in spite of us observing a proton and electron ejected at the neutron's decay. Yet in the same breath they will fight to say that the nucleus contains neutrons because we see them exit the nucleus all the time. They spout the party line, of course, as they must before their peers.

They pity those unwashed who are not learned enough to think that what they actually observe some of the time is not what really happens, yet other times it is OK to observe and say that is what happens. They get to say what is observed is real and what is not real.

Richard Hull

[Chris Bradley]

I always thought a neutron was a proton and an electron (plus a bitsy of pixie dust*)! Again, only a sliver of pedantery separates the rubber stamped version and your own, I would dare to suggest!

(*rubber stamp equivalent = antineutrino)

The rubber stamp physics says a neutron decays into a proton and a W- boson. It is the boson that then splits into the electron and the pixie dust. 0.781MeV worth of pixie dust, I guess, to balance the mass-energy.

[Richard Hull]

You are correct. I just refuse to use the entity neutrino and you with a bit of disdain have dubbed it pixie dust, that is great.....Made me laugh. I look at it as excess binding energy, (mass equivalent) in decay. On the makin' side it is a neutrino or a right handed pinch of pixie dust added and on the decay side it is an antineutrino or a left handed pinch of pixie dust that flies away.

Energy is everywhere in the universe. But it must be remembered that the only reason it is there is that charged mass is always on the move either in accelerated motion EM energy or in uniform motion Magnetic energy. Energy never turns into charged matter and charged matter never turns into energy.

There is the vaporous mass associated with binding energy (fission) and that vaporous mass associated with mass defect (fusion). These latter forms of mass are that mass associated with e=mc^2 in the true e-m and m-e conversion sense.

Richard Hull

[Dustinit]

While this is an interesting line of thought It does not reflect reality as the the suns instability is only due to the proportions of fuel remaining and the total mass of the sun itself. Eventually the hydrogen is converted to heavier elements and the energy released becomes less ( I think iron consumes energy?) and it can no longer support itself against gravity.
Whereas in a fusion reactor the consumed fuel would be constantly removed and gravity is a non issue.
Stability for iter, however, may still only be on the timescale you have suggested,but for other reasons.
Dustin

[Chris Bradley]

Of course, this was only intended as an 'order of magnitude' calculation. But I would question your statement that the sun is unstable due to fuels. Why does it cause instability? This is the question of interest.

In point of fact, accepted stellar evolution - as far as I understand it - has the sun running p-p for 10 billion years (with a proportion of CNO dependent on size and temp, our sun has only a small amount) until there is a build up of He in the core. That's as it becomes a red giant. I have limitied my timing to look at that period. At some point the core can't carry on supporting the pressure as it's buring too cold for anything but p-p, but that's all used up so at some point the star collapses to the point that He+He+He fusion can start up. Then it's a white dwarf.

Now white dwarfs really do have some temperature instabilities, the He fusion being sensitive to temperature fluctuations becuase you have to be pretty lucky or pretty hot and dense to get three nucleii at the same point and time, and all manner of peculiarities then occur - up to and including a stellar collapse and the formation of trans-iron elements that are endothermic in their formation.

The red giant to white dwarf life is another 5 billion or so. 'Tis a trifle to be ignored in my order of magnitude calculation!

[Dan Tibbets]

Comparing Tokamak instability to the sun would best be compared to solar flares- Coronal mass ejections, not the red giant stage. The red giant stage is a fuel issue, the aviable hydrogen is used up, the core cannot support itself against gravity and it is compressed-and heated by the compression untill the helium ash starts to burn. As the core is now smaller, but much hotter it heats the overlieing gas and like any gas as it is heated it expands - into the red giant size. Also, a white dwarf is a dead star. It has burned all the light elements it can- up to ~ iron ( actually most only have enough mass to fuse up to carbon or posibly oxygen. The star in its red giant stage has ejected most of the outer gas envelope and just the core is left. When a white dwarf star is young this exposed core surface is at ~ 100,000 degrees C and emits copius amounts of UV light - which is what energies the planetary nebula that formed during the red giant stage. The white dwarf then cools passively ( black body radiation). Unless the white dwarf has a close companion star that it can steal matter from -leading to novas or a type of supernova, the white dwarf will gradually cool and dim over billions of years till it is just a lump of 'coal'.
Actually, the Sun is fairly stable as stars go (fortunately for us). Instead of the Sun as an an example for instability, red dwarfs are better. Some tend to be much more unstable. Some can change in brightness by several hundred fold within days, or even hours .
http://en.wikipedia.org/wiki/Flare_star

And the big boys like Eta Carni can occasionally burp and eject multiple solar masses worth of matter within days.


Dan Tibbets

[Dustinit]

Its interesting that you say its a lump of coal,
I recall that once it was seriously considered early on that it was coal that fuelled
the sun but I think it was discarded because it wouldnt burn for long enough.
Dustin

[Chris Bradley]

Sure. All agreed.

I'll go further than that to look at the differences:

The sun versus tokamak;
different volume, by 27 orders of magnitude,
different pressure, by 14 orders of magnitude, (equivalent to 28 orders of magnitude rate reactions)
different temperature, by only one order of magnitude, (but reaction rate is a negative exponential function, could we call that e^-10 = 5 orders of magnitude?)
different fuel, giving a different fusion probability 25 orders of magnitude difference,
different shape,
different containment mechanism,

..have I missed anything..?

..not a tiniest scrap of similarity. Complete nonsense to draw any similarities between a tokamak and a star (as so often poetically described in the public domain).

(In fact, is there ANY unique distinguishing feature that is common to the two processes that is not common between some other two processes??)

We're looking at a 'sum total' of 85 orders of magnitude difference - some plus, some minus.

All the more odd, then, that relative overall 'burntime' between the two can be predicted using these simple assumptions of instability within just one order of magnitude!!??

Maybe it's just co-incidence, of course, but co-incidence isn't the first thing I think about when the maths supports an argument. Instead I start wondering what else to look for that might show it was just co-incidence, before dismissing it.

best regards,

Chris MB.

[Richard Hull]

The Sun and the Tokamak both do fusion via a thermal process of heating the fuel. (maxwellian devices) Just about the only similarity.

Richard Hull

[Chris Bradley]

I'd suggest even those two points are arguable.

The sun heats itself by its own reaction, initiated by a one-time gravitational contraction and sustained by being well-insulated.

The Tokamak requires a continuous energy input to maintain the heat and looses all the energy it produces in neutrons that escape the plasma.

So one *is* a thermal process, the other *is acted on by* thermal processes.

The second point is a bit more obscure - I note recent published research that shows as temperature increases to earthbound tokamak levels, the thermal distribution drifts away from Maxwellian into an inverse 8th power function due to quantum effects.

[Mike Beauford]

That's interesting about the 8th power. I think I recall Dr. Bussard said something similar I thought when he was describing how his pollywell would size vs power.

[Chris Bradley]

I couldn't say if that is a co-incidence or not! The derivation of the inverse 8th power momentum distribution is the work of Galitskii and Yakimets. With names like that, I expect it is easy to search for them (!), if such details of plasma dynamics is of interest. I don't know much about this, only that I came across it whilst researching non-maxwellian distributions.

[Dan Tibbets]

Indeed there is little similarity except the fusion reactions themselfs (hopefully). But, in terms of the plasmas and their reactions, the studies of low to high mass stars, neutron stars, black holes , etc might give insights into the interactions of plasmas, and magnetic fields on the Earth (and visa versa). And, while still driven by gravity, Novas are nuclear detonations on the surfaces of white dwarf stars that have accumulated enough hydrogen from a close companion.
http://en.wikipedia.org/wiki/Nova
How the star accumulates enough hydrogen and compresses/heats it to ignition conditions without first ejecting the hydrogen in a powerful solar wind is interesting. From an idiots perspective I can speculate on various gravity gradiants, companion star feed rates, sound waves/ shock wave effects, and magnetic effects that might apply. And to drift further off topic, if the white dwarf star accumulates enough hydrogen from a companion star to blow off a portion of it in a Nova periodically, how can it go past that point to where enough mass is accumulated to trigger the fusion of the entire core's content of carbon/oxygen into iron - type 1a Supernova?

Dan Tibbbets

[Dissident]

I'm more interested how Jet aircrafts can be replaced with plasma saucers - something like the "Lightcraft" of NASA. When??

[Carl Willis]

Lubomir, you're in the wrong place to carry on like this. You need to keep your additions topical (i.e. the science of nuclear fusion) or you'll find your postings shuffled off to the troll repository.

Thank you--in advance--for playing by the rules.